1. Field of the Invention
The present invention relates to a matching circuit used for a circuit element such as an amplifier and, in more specifically, to a matching circuit capable of matching the input/output impedance of a circuit element, such as an amplification device, that has a frequency characteristic to a particular impedance in each of two or more frequency bands.
2. Description of the Related Art
With the diversification of services provided by radio communications, multiband operation of radio devices is required. Among essential devices included in radio devices is a power amplifier. For an efficient amplification, a matching circuit for matching the input/output impedance of an amplification device having a frequency characteristic to the input/output impedance Z0 of peripheral circuitry (hereafter referred to as the system impedance Z0), namely a multiband matching circuit, is required.
FIG. 1 shows an example of input/output scattering parameters (S parameters) of an amplification device, where the upper limit of measured frequencies is denoted by fmax and the lower limit is denoted by fmin. In FIG. 1, S11 is the input reflection coefficient of the amplification device at an output impedance of 50Ω and S22 is the output reflection coefficient of the amplification device at an input impedance of 50Ω. The input/output impedance of the amplification device can be obtained from the S parameters and the system impedance Z0. Accordingly, as apparent from FIG. 1, the input/output impedance of the amplification device has a frequency characteristic. Therefore, the input/output impedance of the amplification device is represented by a function, ZL(f), as an input/output impedance that is dependent on frequency f. A multiband matching circuit is a circuit that matches the impedance of a circuit element having a frequency characteristic, such as an amplification device, to the system impedance Z0 in each of desired frequency bands. In the following description, an amplification device will be adopted as a representative example of circuit elements whose impedances have a frequency-dependent characteristic.
Typical matching circuits are often formed by a combination of circuit elements such as a capacitor and an inductor. However, it is not straightforward to form a multiband matching circuit by using circuit elements because each circuit element has a unique frequency characteristic. Therefore, for example there are two possible ways to configure an amplifier that operates in different frequency bands: [1] a method in which amplifiers, each designed for any one of the predetermined frequency bands, are provided and any one of the amplifiers are selected by switches corresponding to an operating frequency band of the predetermined frequency bands, and [2] a method that takes into account the characteristic of an amplification device that can amplify wideband signals, in which a single amplification device and a matching circuit capable of changing a circuit constant thereof are provided, and the circuit constant of the matching circuit is changed in accordance with operating frequency bands. As the method [2], approaches have been proposed in which low-loss switches and variable-capacitance elements developed in recent years are used to change the circuit constant of the matching circuit.
An example of [1] is a power amplifier described in non-patent literature 1 (Koji Chiba et. al, “Mobile Terminals”, NTT DoCoMo, Vol. 4, No. 1, pp. 14-19). FIG. 2 shows a circuit configuration of a power amplifier (dualband power amplifier 900) capable of amplifying each signal in two frequency bands. The center frequencies of the two operating frequency bands are f1=1.5 GHz and f2=0.8 GHz. Matching circuits generally can perform impedance matching for signals in a certain frequency band around a certain center frequency. The dualband power amplifier 900 includes an amplifier 921 designed specifically for a frequency band with center frequency f1, and an amplifier 922 designed specifically for a frequency band with center frequency f2, as shown in FIG. 2. In the dualband amplifier 900, a single-pole double-throw (SPDT) switch 911 connected to an input terminal 931 and an SPDT switch 912 connected to an output terminal 932 are turned on or off depending on which of the center frequencies f1 and f2 is used, thereby selecting one of the amplifiers 921 and 922.
However, the concept of [1] requires as many amplifiers as the number of frequency bands to be used. If many frequency bands are to be used, a large number of components must be provided, resulting in a large circuitry size. The increase in the number of components leads not only to a large device size but also to an increase in power consumption in the entire circuitry.
An example of [2] is a matching circuit 950 disclosed in non-patent literature 2 (Fukuda et. al, “Multiband Power Amplifier Employing MEMS Switches for Optimum Matching”, Proceedings of Institute of Electronics, Information and Communication Engineers General Conference 2004, C-2-4. p. 39) (see FIG. 3). The matching circuit 950 includes a main matching block 951, a delay circuit 952 one end of which is connected to the main matching block 951, an auxiliary matching block 953 connected to the other end of the delay circuit 952 through a switching element 954.
The matching circuit 950 shown in FIG. 3 matches the impedance ZL(f) of a load (a circuit element) 955 connected to port P2 to the constant impedance Z0 of a load (a circuitry) 956 connected to port P1 and functions as a matching circuit for each of signals in two frequency bands with center frequencies f1 and f2, for example. The mechanism of the operation will be described below.
First, for impedance matching in the frequency band with center frequency f1, the switching element 954 is turned off (brought into the non-conduction state). The main matching block 951 is a circuit that converts the impedance ZL(f1) of the load 955 to the impedance Z0 of the load 956 for a signal in the frequency band with center frequency f1. Here, by implementing the delay circuit 952 by, for example, a transmission line of the characteristic impedance Z0, impedance matching by the matching circuit 950 as a whole can be achieved for the frequency band with center frequency f1 because the impedance as viewed from port P1 toward point A shown in FIG. 3 becomes Z0.
For impedance matching in the frequency band with center frequency f2, the switching element 954 is turned on (brought into the conducting state). The main matching block 951 designed as described above functions as an impedance converter for a signal in the frequency band with center frequency f2 and the impedance as viewed from point A toward port P2 becomes Z(f2) converted from the impedance ZL(f2) of the load 955. Here, the impedance as viewed from port P1 toward port P2 can be converted to Z0 on the basis of the principle of single stub matching by choosing an appropriate line length of the delay circuit 952 implemented by a transmission line and an appropriate reactance value of the auxiliary matching block 953 branch-connected to the transmission line whatever value the impedance Z(f2) has. That is, in the matching circuit 950 as a whole, impedances can be matched for the frequency band with center frequency of f2 as well.
With the configuration of the matching circuit 950 which comprises the delay circuit 952 of the characteristic impedance Z0 and the auxiliary matching block 953 connectable by the switching element 954 as well as the main matching block 951 which is a matching circuit for the frequency band with center frequency f1, the matching circuit 950 functions as a dualband matching circuit for a signal in the frequency band with center frequency f1 when the switching element 954 is in the off state and for a signal in the frequency band with center frequency f2 when the switching element 954 is in the on state. That is, the matching circuit 950 functions as a matching circuit for each of the signals in the two frequency bands by turning on or off the single switching element 954.
In the matching circuit 950, the amount of delay in the delay circuit 952 needs to be increased, depending on the frequency characteristic of the input/output impedance of an amplification device. If the delay circuit 952 is implemented by a transmission line, the matching circuit 950 will increase in size because the length of the transmission line is proportional to the amount of delay. The size of the matching circuit 950 will be more likely to increase because a plurality of delay circuits and auxiliary matching blocks are provided if the number of the operating frequencies is three or more. By the way, a circuit equivalent to the delay circuit 952 could be formed by a set of small lumped-parameter elements. However, it is very difficult to design a delay circuit that is capable of adjusting the amount of delay under the condition that the impedance Z0 should be sustained in each of multiple frequency bands. For these reasons, reduction of the size (shortening of the transmission line) of the part equivalent to the delay circuit is required in order to reduce the size of the multiband circuit in the matching circuit as shown in FIG. 3.